THE OCEAN 339 



In many harbors, especially where the water is shallow, the 

 rise and fall have an important effect on navigation. Vessels 

 coming to such harbors at low tide must often wait until high 

 tide before entering. Where the tide runs in among islands, or 

 passes through narrow straits, it often causes distinct currents, or 

 tidal races. They are sometimes so strong as to interfere with boats, 

 especially small ones. 



Tides are not felt in small lakes, and are feeble in large lakes, 

 enclosed seas, and in all bodies of water connected with the open 

 sea by a narrow passageway, such as the Mediterranean Sea and 

 the Gulf of Mexico. Thus at Galveston, Texas, the range of the 

 tide is less than one foot. 



The tide sometimes runs up a broad open river. As it advances 

 up the channel, its front may become a steep, wall-like wave called 

 a bore. The bore is felt, for example, in the Severn River of Eng- 

 land, in the Seine of France, in the Tsien-Tang-Kiang of China, etc. 

 In the last-named river the waves are sometimes 25 feet high, and 

 disastrous to navigation. High tides are felt, though not as bores, 

 up the Hudson River to Troy, where the range of the tide is more 

 than two feet. Tides are felt up the estuary of the St. Lawrence 

 283 miles, nearly to Montreal. 



The periodicity and the cause of tides. The moon rises and sets 

 twenty-four hours and fifty-two minutes later each day than it 

 did the day before. The time between two high tides or between 

 two low tides is half this period. It appears to have been this 

 fact which suggested that the tides were caused by the moon. It 

 is at least two thousand years since the moon was first thought to 

 be the cause of the tides, but it is only about two hundred years 

 since Newton explained how the moon produces this result. 



The law of attraction between bodies has already been stated 

 (p. 333). Without attempting to give a full explanation of the 

 tides, some of the principles involved may be understood. 



The earth and the moon attract each other, and would fall 

 together but for the centrifugal force due to their motions. This 

 centrifugal force may be illustrated as follows: At the center of 

 the earth, and at the center of the moon, the attraction between 

 these bodies is exactly balanced by the centrifugal force due to 



